The present invention relates to an overdrive system for a four-wheel drive vehicle having an automatic transmission and a clutch which is manually or automatically engaged to transmit power of an engine to auxiliary driving wheels for establishing a four-wheel driving power transmission.
Generally, the four-wheel driving is used when the vehicle is driven on slippery or icy or snowy roads for safety driving, and the overdriving is hardly used during such four-wheel driving. Accordingly, the overdrive system is provided for two-wheel driving, and it is not necessary to provide for four-wheel driving.
On the other hand, an overdrive device which is directly coupled to the crankshaft of an engine by a lock-up device of a torque converter is desirable for fuel economy. However, in such a system, a lock-up device and a system for operating the lock-up device must be installed, which causes complicated mechanical construction and complex control system. Japanese patent application laid open No. 57-61843 discloses an overdrive system comprising a pair of speed up gears provided on an oil pump drive shaft inherently installed in the automatic transmission and a clutch for establishing the overdrive. However, it is difficult to employ such an overdrive system in a four-wheel drive vehicle which is provided with a transfer device including a clutch for establishing the four-wheel driving, since the transfer device is positioned at a place for installing the overdrive system.